score:6
So - to answer my own question:
Yes this does happen.
I've been checking interview appointment availability on the consulate web site twice a day since I made the original appointment, and today saw some new slots open up in the date range I was aiming for and I've now rescheduled for the 19th.
Upvote:3
Is there any real likelihood that I will see some appointment dates open up later in the month if I wait a few days or should I cut my losses now and make other arrangements ... ?
Since there is likelihood there is no need to bother with other arrangements. 'Push comes to shove' and you still have 10 days anyway.
According to the US Consulate visa call-center in SA:
the consulate had not yet released their full availability for December
It is not unusual in general for additional appointment slots to become available with time and some of the reason for this is considered at Taking someone else's cancelled German visa appointment slot - realistic?.